S2E2 - Discovery of the Electron by J.J. Thomson, and the Discovery of the Nucleus by Ernest Rutherford

SECTION 2 - Atoms Molecules and Ions

Discovery of the Electron

The Electron  =  discovered in 1903 by J.J. Thomson as he conducted experiments with a cathode ray tube.


J.J. Thomson's Cathode Ray Tube

As you can see in the image below, cathode rays are streams of electrons running through the cathode ray tube from cathode to anode.

These cathode rays are produced by the applied voltage between the two electrodes.

When an outside electric field is applied (image on the right), we can see that the electrons (cathode rays) are attracted to the positive end of the applied electric field...

J.J. Thomson's Cathode Ray Tube

Cathode Ray Tube Experiment

➞ Summary of J.J. Thomson's results:

1.  Because the "cathode rays" were deflected away from the negative end of an applied electric field, Thomson postulated that the cathode rays are negatively-charged particles called electrons.

2.  Because atoms were known to be neutral, Thomson reasoned that there must be a positive charge somewhere in the atom as well.


Plum Pudding Model of the Atom

J.J. Thomson's Cathode Ray Tube Experiments inspired him to propose his "Plum Pudding Model" of the Atom.

See the image below...

Plum Pudding Model of the Atom
In his Plum Pudding Model, Thomson assumes the electrons are spread out like raisins in a pudding:

➞ tiny, negative, point-charges (raisins) arranged in a spherical cloud (pudding) of positive charge.


NOTE  -  We are not mentioning the word "protons" here.

They are NOT discovered yet !!


The Charge and Mass of the Electron

1909 - Robert Millikan performed experiments that determined the charge of an electron and the mass of an electron.

➞ The charge of an electron:  1.6 x 10-19 C

➞ The mass of an electron:  me  =  9.1 x 10-31 kg


Rutherford's Gold Foil Experiment

1911 - Ernest Rutherford disproved Thomson's "Plum Pudding Model" of the atom and discovered the nucleus.

Rutherford's experiments involved bombarding various atoms of gold foil with low-energy alpha particles (α-particles):

Rutherford's Gold Foil Experiment

Summary of the Gold Foil Experiment:

➞ most alpha-particles pass directly through because the atom is mostly open space.

➞ deflected particles are those that had come "close to" the positively-charged center.

➞ reflected particles had a "direct hit" with the center.


Discovery of the Nucleus

The densely-packed center of the atom is called the nucleus and represents 99.99% of an atom's mass.

How dense is the nucleus you ask?? -- If a nucleus was the size of a grain of sand, it would weigh 11 million pounds!


Next up in our discussion of SECTION 2 - Atoms, Molecules, and Ions,

We'll answer the question:

What are Isotopes and What is Modern Atomic Structure?